Magneto-resistance (MR) is a material property of a whole family of ferromagnetic alloys that refers to a dependence of electrical resistance on the angle between the direction of the electrical current flowing through the material and the orientation of an external magnetic field relative to the direction of the current. The effect is attributed to a larger probability of s-d scattering of electrons in the direction of the magnetic field. The net effect is that the electrical resistance maximum value when the direction of current is parallel to the applied magnetic field. An example of such material is a ferromagnetic material called “permalloy” (19% Fe, 81% Ni).
MR materials can be used to create magnetic field sensors, also referred to as magneto-meters. Operation and examples of such sensors are described in Application Note AN 00022 “Electronic Compass Design using KMZ51 and KMZ52”, author Thomas Stork, of Philips Semiconductors, dated Mar. 30, 2000. The KMZ52 is a commercially available electronic device manufactured by Philips that comprises the components of a compass sensor system within one package: two weak-field sensors with 90° displacement, each having a set/reset (flip) coil and a compensation coil. Typical current levels are 10 mA for the compensation coil and 1 A for the flip coil. About 2 mA is sufficient to balance the earth magnetic field. Therefore, the resistance of the flip coil is preferably relatively low, e.g., in the order of a few Ohms. Such sensors are manufactured using e.g., a thin-film technology or an integrated circuit technology.
Magnetic field sensors can be used in e.g., solid state compassing, metal detection, position detection, etc.
First, consider a sensor made of a simple strip of MR material. During fabrication, a strong external magnetic field is applied parallel to the strip's main axis. As a result, a preferred magnetization direction is defined in the strip. In the absence of a magnetic field, the magnetization always points into that direction. The operation of the sensor relies on two effects. The first effect is that the resistance of the strip depends on the angle between a direction of the current flowing through the strip and the direction of the magnetization. The second effect is that the direction of the magnetization, and therefore of the angle, can be influenced by an external magnetic field parallel to the strip and perpendicular to the preferred direction.
The simple strip sensor has a low sensitivity for small magnitudes of the external magnetic field. In addition, the simple sensor cannot discriminate between external magnetic fields of the same magnitude but with opposite directions. Therefore, the sensor has preferably a so-called “barber-pole” configuration. This is achieved by depositing e.g., aluminum stripes (called “barber poles”) on top of the MR strip at an angle of 45° to a main axis of the strip. As aluminum has in general a much higher conductivity than MR material, the effect of the barber pole is to rotate the current direction by 45°, effectively changing the angle between the magnetization of the MR material and the electrical current from an angle of magnitude “α” to an angle of magnitude “α−45°”. For weak magnetic fields such as the earth's field, the sensitivity now is significantly higher. In addition, the characteristic is linearized and allows detecting the sign of the external magnetic field.
In practice, it is advantageous to configure the sensor as a Wheatstone bridge, consisting of four magneto-resistive strips. For e.g., compass sensors, the barber pole structures are used, where one diagonal pair is orientated at +45° to the strip's main axis, and the other pair is orientated at −45°. Thus, the resistance variation due to a magnetic field is converted linearly into a variation of the differential output voltage. Moreover, the inherent temperature coefficients of the four bridge resistances are mutually compensated.
MR sensors are bi-stable by nature. That is, the direction of their internal magnetization can be inverted or “flipped”. This can be achieved by a magnetic field of sufficient strength, if that field is applied parallel to the magnetization, but having opposite direction. Flipping causes an inversion of the sensor characteristic, such that the sensor output voltage changes polarity. MR sensors can be stabilized against unwanted flipping by applying an auxiliary magnetic field parallel to the flipping axis. This auxiliary field should be pulsed, as a permanent field would decrease the sensitivity of the magnetometer. When measuring weak fields, it is even desired to invert or “flip” the sensor characteristic repetitively between consecutive magnetometer readings. This allows compensating the sensor's offset drift in a way comparable to the chopping technique used in the amplification of small electrical signals. A “set/reset” coil also referred to as “flip” coil, near the sensor element is a means to apply the auxiliary field for the flipping. In e.g., high-precision compass systems, the sensor must also allow to compensate for sensitivity drift with temperature and to compensate for interference fields. Both can be done by means of an auxiliary field in the field-sensitive direction that is perpendicular to the MR strips. This can be generated by a “compensation” coil near the sensor element.
An example of an electronic device with MR sensor strip electronic device with an MR sensor that comprises an MR sensor strip, a flip coil functionality and a compensation coil functionality is known from WO 99/09427. The known device has sensor strips all oriented in the same direction, and included in a Wheatstone bridge. The set-reset coil, or flip coil, is oriented perpendicular to the strip direction and compensation coils are oriented in the direction of the strips.
Another example is known from DE 196 48 879. In the known device, the flip coil consists of a combination of different current lines over a sensor strip. The current density on the outer current lines is higher than in the inner lines. This allows lowering of the switching threshold of the magnetic sensor strip.